This invention relates generally to electrical machines and more particularly, to methods and apparatus for operating electrical machines.
Generally, a wind turbine generator includes a turbine that has a rotor that includes a rotatable hub assembly having multiple blades. The blades transform mechanical wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are generally, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbine generators also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a base that may be a truss or tubular tower.
Some gearless direct drive wind turbine generator configurations include doubly fed induction generators (DFIGs). Such configurations may also include power converters that are used to transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. Under certain circumstances, grid voltage fluctuations may be experienced that may include low voltage transients with voltage fluctuations that approach zero volts. Generally, the power converters and the generator are susceptible to grid voltage fluctuations. Therefore, such grid voltage fluctuations may be deleterious to continuous operation of the wind turbine generator.